Cryopumps currently available, whether cooled by open or closed cryogenic cycles, generally follow the same design concept. A low temperature array, usually operating in the range of 4 to 25 K, is the primary pumping surface. This surface is surrounded by a higher temperature radiation shield, usually operated in the temperature range of 70 to 130 K, which provides radiation shielding to the lower temperature array. The radiation shield generally comprises a housing which is closed except at a frontal array positioned between the primary pumping surface and the chamber to be evacuated. This higher temperature, first stage frontal array serves as a pumping site for higher boiling point gases such as water vapor.
In operation, high boiling point gases such as water vapor are condensed on the frontal array. Lower boiling point gases pass through that array and into the volume within the radiation shield and condense on the lower temperature array. A surface coated with an adsorbent such as charcoal or a molecular sieve operating at or below the temperature of the colder array may also be provided in this volume to remove the very low boiling point gases such as hydrogen. With the gases thus condensed and/or adsorbed onto the pumping surfaces, only a vacuum remains in the work chamber.
Once the high vacuum has been established, work pieces may be moved into and out of the work chamber through partially evacuated load locks. With each opening of the work chamber to the load lock, additional gases enter the work chamber. Those gases are then condensed onto the cryopanels to again evacuate the chamber and provide the necessary low pressures for processing. After continued processing, perhaps over several weeks, gases condensed or adsorbed on the cryopanels would have a volume at ambient temperature and pressure which substantially exceeds the volume of the cryopump chamber. If the cryopump shuts down, that large volume of captured gases is released into the cryopump chamber. To avoid dangerously high pressures in the cryopump with the release of the captured gases a pressure relief valve is provided. Typically, the pressure relief valve is a spring-loaded valve which opens when the pressure in the cryopump chamber exceeds about 3 pounds per square inch gauge. Because the process gases may be toxic, the pressure relief valve is often enclosed within a housing which directs the gases through an exhaust conduit.
After several days or weeks of use, the gases which have condensed onto the cryopanels and, in particular, the gases which are adsorbed begin to saturate the system. A regeneration procedure must then be followed to warm the cryopump and thus release the gases and to remove the gases from the system. As the gases are released, the pressure in the cryopump increases and the gases are exhausted through the pressure relief valve.
A typical pressure relief valve includes a cap which, when the valve is closed, is held against an o-ring seal by a spring. With pressures which open the valve, the cap is pushed away from the o-ring seal and the exhausted gases flow past the seal. Along with the gas, debris such as particles of charcoal from the adsorber or other debris resulting from processing within the work chamber also pass the seal. That debris often collects on the o-ring seal and the closure cap. In order to effect a tight vacuum after regeneration it is often necessary to clean the relief valve after each regeneration procedure. If such care is not taken, leaks into the cryopump result at the relief valve and provide an undesired load on the cryopump.
After the gases have been released from the cryopanels and cryopump chamber, a vacuum is again created in the cryopump. Before cooling the cryopump to cryogenic temperatures, however, the cryopump must first be rough pumped to remove essentially all water vapor from the cryopump chamber and reduce the pressure in the chamber to a level at which the cryopump may operate. A valve is positioned between the cryopump chamber and the rough pump and that valve is also subject to contamination from debris. As with the pressure relief valve, such contamination can result in leaks which prevent effficient operation of the cryopump.
Several approaches have been suggested for contamination. One approach has been to provide a self-cleaning valve which isolates the o-ring in a relief valve from contaminants. Such an approach increases the complexity of the relief valve. Another approach has been to position a filter in the exhaust conduit to the relief valve. However, to eliminate the danger of an extreme pressure buildup in the case of clogging of the filter, a pressure relief of the filter itself is required. Another approach has been to prevent the debris from reaching the exhaust conduit by positioning a stand pipe at the opening to the conduit. The standpipe causes the debris to collect at the bottom of the cryopump housing. An equivalent arrangement is to position the exhaust port along the sidewall of a cryopump chamber rather than at its base. These latter approaches result in the collection of liquid cryogens and water in the cryopump chamber during the regeneration process and thus significantly increases the regeneration time and the rough pumping time subsequent to regeneration.